It is known in the art to overcome the problems of slow filtration rate, rapid medium binding and low filtration efficiency by means of filtration aids. Filtration aids generally comprise granular or fibrous materials, for example diatomaceous earth, which are capable of forming highly permeable filter cakes upon which filtration takes place.
Such filtration aids are generally only used in batch filtration processes wherein filtered matter progressively builds up in cake form until an unacceptably high pressure drop develops over the cake. The filtration process must then be interrupted in order to remove the cake either by backwashing or opening of the filter followed by physical removal of the cake. The filtration aid, almost invariably, cannot be separated from the filtered matter and is lost resulting in high operating expenditure. One commonly used filter aid is diatomaceous earth which is the skeletal remains of aquatic plants originating in prehistoric times and which is very costly.
It will be understood by those skilled in the art that the terms "cake" or "filter cake" imply a packed layer of filtered matter or contaminants which is produced. adjacent the filter medium of a filter, by the progressive deposition of the filtered matter thereon. It will further be understood by those skilled in the art that the term "filtration cycle" implies the filtration process from one shutdown to the next and may include the steps of pre-coating a substrata with filter aid, filtering and backwashing or cleaning to remove both filter aid and filtered solids from the filter.
It has been suggested that, in order to prolong the filtration cycle between successive backwashings, filter aid be added in small amounts during filtration. This, however, increases the consumption of filter aid in any event
It is accordingly an object of this invention to provide a method of batch filtration and control means to effect the method of filtration which at least increases the period of filtration between successive backwashings by increasing the amount of solids that can be filtered out using a predetermined amount of filter aid, thereby decreasing operating costs.